1. Field of the Invention
The present invention relates to a method and apparatus of handling hybrid automatic repeat request (HARQ) operation for semi-persistent scheduling (SPS), and more particularly, to a method and apparatus of handling HARQ operation for SPS in a user equipment (UE) of a wireless communication system, so as to avoid unexpected data combination.
2. Description of the Prior Art
Long Term Evolution wireless communication system (LTE system), an advanced high-speed wireless communication system established upon the 3G mobile telecommunication system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in Node B (NB) alone rather than in NB and RNC (Radio Network Controller) respectively, so that the system structure becomes simple.
The LTE system has two scheduling methods: dynamic scheduling (DS) and semi-persistent scheduling (SPS). For DS, the network dynamically allocates resources to UEs for data reception or transmission depending on traffic volume, quality of service (QoS) requirements of each UE. And for SPS, the network periodically allocates resource to UEs, in order to serve upper layer applications which generate semi-static size data periodically, e.g. Voice over Internet Protocol (VoIP) services, for reducing control information sent on a physical downlink control channel (PDCCH) and enhancing system scheduling performance. In other words, SPS provides persistent transmission resources to the UE, such that the UE can perform periodic data transmission without monitoring PDCCH and make sure each periodic transmission is a new transmission.
The resources assigned by DS are addressed to a Cell Radio Network Temporary Identifier (C-RNTI) of the UE, while the resources assigned by SPS are addressed to an SPS C-RNTI of the UE. The SPS C-RNTI is a UE specific identity assigned by Radio Resource Control (RRC) layer, and is manly used for activating or modifying SPS resources and for indicating SPS retransmissions.
For DS, a UE recognizes that the assigned resource is for a new transmission or a retransmission by HARQ Process ID and New Data Indicator (NDI) included in the PDCCH signaling. If the value of NDI is different from a previous NDI of the same HARQ process, the assigned resource is for a new transmission. Conversely, if the value of NDI is the same as the previous NDI of the same HARQ process, then the assigned resource is for a retransmission.
However, the use of NDI in SPS is different from that in DS. For SPS, the value of NDI is used for indicating the UE the purpose of PDCCH signaling addressed to its SPS C-RNTI. If the value of NDI is 0, it means that the PDCCH signaling addressed to UE's SPS C-RNTI is used for activating or modifying SPS resources. Conversely, if the value of NDI is 1, then the PDCCH signaling addressed to SPS C-RNTI is used for allocating transmission recourse of an SPS retransmission.
According to current specifications, in downlink direction, one or more specific HARQ process ID(s) reserved for SPS is configured by RRC signaling, but in uplink direction, the HARQ process ID(s) reserved for SPS is recognized after configuring start time and cycle length of an SPS period. However, DS is also allowed to use the HARQ process ID(s) reserved for SPS both in downlink and uplink direction. In this situation, when a UE sequentially receives two NDIs of a same HARQ process which are included in a PDCCH signaling addressed to SPS C-RNTI and C-RNTI, respectively, since the meaning of NDI field are different for DS and SPS, comparing the two NDIs is meaningless, and may cause some unexpected data combination due to the comparison result.
For example, when the UE receives a PDCCH signaling addressed to its SPS C-RNTI, which indicates the UE to use some HARQ process, such as X to perform SPS retransmission, based on the above illustration, the value of NDI shall be 1. Next, the UE receives a PDCCH signaling addressed to its C-RNTI, which indicates the UE to use the HARQ process X to transmit data as well. In this case, the UE compares the DS NDI with the previous NDI of the HARQ process X, i.e. the SPS NDI. However, the meaning of SPS NDI is different from that of DS NDI. Thus, comparing the two NDIs is meaningless, and may cause some unexpected data combination due to the comparison result.